There is an unfilled need for improved, nondestructive means to test bulk dielectric materials for flaws, defects, irregularities, and other features; and particularly to determine the absolute thickness of bulk dielectric materials when given access to only one side of a part under inspection. Additionally, there is an unfilled need for improved, nondestructive means to determine variations in the density (or porosity) when the thickness of a bulk dielectric material is known. For example, there is an unfilled need for improved, nondestructive means for examining dielectric materials in three dimensions, volumetrically, and measuring both thickness and changes in thickness. For a manufactured dielectric component that has been in service for some time, the remaining thickness is often important as an indicator of the component's remaining life; but it can be difficult to measure thickness when only one surface of the component is accessible. Density can also be a major indicator of the serviceability of manufactured dielectric components, because the density often relates directly to the strength of the component. The dimensions of a manufactured part are often known or are easily measured, but it is more difficult to determine density and variations in density. There is an unfilled need for improved means for the nondestructive determination of density and changes in density of a bulk dielectric material when its thickness is known.
For example, there is an unfilled need for enhanced, nondestructive means for measuring the remaining wall thickness in dielectric tanks and pipes. (This invention has numerous applications, and is not limited to the inspection of tanks and pipes.)
Modern chemical processing often involves the use of components made of dielectric materials. Common dielectric material product forms include fiber reinforced plastic (often called “fiberglass” or “FRP”) pipes and vessels. These materials are also commonly used in modern infrastructure, such as drinking water and waste water processing. There exists an unfilled need for improved means to measure the thickness of such materials nondestructively, especially for means that may be employed while the produce remains in-service, and where there is access to only one side of the dielectric component. (This invention has numerous applications, and is not limited to the inspection of FRP.)
Due to the corrosive or abrasive nature of the fluids that are often used in various processes, the wall thickness often diminishes over time as a direct result of service-induced degradation. These service-induced thickness changes are difficult to detect nondestructively through conventional means.
It is highly desirable that a testing method should be nondestructive, and that it should be usable whether a facility is running or idle. Furthermore, because the access space outside the component can be limited, and the geometry of a component can be complex, any portion of the detection machinery that must be in contact with the component (or in the vicinity of the component) should be small enough to accommodate the available space and geometry.
When the component to be tested is made of metal, then well-established ultrasonic inspection techniques can be used. However, ultrasonic inspection cannot be used effectively for reinforced dielectric materials, because the structural fibers scatter nearly all sound energy, and return little usable signal. The mesh or fabric of a composite material so strongly scatters and disperses ultrasonic waves that the resulting reflection is extremely noisy. Eddy current measurements or magnetic measurements do not work well in these materials either, because they do not conduct electricity.
Neither is radiography particularly helpful. X-ray radiography can be used to detect changes in bulk density or to detect changes in thickness, but it requires access to both sides of the component under inspection. This renders X-ray radiography ineffective for in-service inspection of many components.
Another example of an unfilled need for improved methods to measure density lies in the field of engineered ceramic composites. In such composites both the reinforcing fibers and the matrix are made of a ceramic material. Typically, the fibers are woven or otherwise arranged into a support structure into which the matrix is deposited by chemical methods. The matrix is typically deposited iteratively. The chemical reaction that results in the deposition occurs in sequential steps, with each step depositing additional ceramic material into the interstices between reinforcing fibers. Since the location of the fibers and the outer boundary of the part do not change, the porosity of the part decreases (and its density correspondingly increases) with each iteration. When the parts are highly engineered and their physical dimensions are closely controlled, the physical thickness, measured in inches or mm, is generally known within close tolerances. Because the strength of a part is typically a function of its density, it would be highly desirable to have improved nondestructive means to measure density. Ultrasonic methods are not effective for determining density in such materials, due to the scattering of sound waves by the reinforcing fibers. Neither can eddy current or magnetic methods be used, as the ceramic composites are bulk non-conductors. Although changes in density can be detected by radiography, the changes of interest in ceramic composite applications are typically too small to be resolved by conventional radiography. Additionally, radiography requires access to both sides of the part, for that reason is not an acceptable method in many circumstances.
An overview of microwave testing techniques is given in A. Bahr, Microwave Nondestructive Testing Methods (1982).
Several microwave nondestructive testing techniques are disclosed in A. Lucian et al., “The Development of Microwave NDT Technology for the Inspection of Nonmetallic Materials and Composites,” pp. 199-232 in Proceedings of the Sixth Symposium on Nondestructive Evaluation of Aerospace and Weapons Systems Components and Materials (San Antonio, Tex. 1967).
J. Kurian et al., “Microwave Non-Destructive Flaw/Defect Detection System for Non-Metallic Media Supported by Microprocessor-Based Instrumentation,” J. Microwave Power and Electromagnetic Energy, vol. 24, pp. 74-78 (1989) discloses a method for detecting defects in a tire by measuring transmission of microwaves from a dipole transmitting antenna inside the tire, through the treads of the tire, with transmission detected by a linear array of detectors. Differential rates of transmission were correlated with changes in thickness or with defects.
C. Howell et al., The Use of Low Cost Industrial AM-CW ‘Microwave Distance Sensors’ for Industrial Control Applications (no date) discloses a microwave distance sensor to measure distances to an object from about 15 centimeters to about 6 meters away, by measuring the phase angle of a returned amplitude-modulated microwave signal reflected from the object.
U.S. Pat. No. 3,278,841 discloses a microwave flaw detection system, particularly for use with large, solid-propellant rocket motors. Microwaves were transmitted from inside the propellant, reflected off a metal casing, and detected by a receiver displaced from the microwave transmitter. Irregularities in the strength of the received signal were correlated with cracks or other flaws in the propellant.
U.S. Pat. No. 4,520,308 discloses a system for measuring the thickness of a dielectric material by measuring the phase shift of microwaves transmitted along a microwave strip line conductor adjacent to the material whose thickness is being measured. See also U.S. Pat. No. 4,123,703.
U.S. Pat. No. 2,999,982 discloses a Doppler-effect-based method for microwave detection of inhomogeneities in compact materials such as polished glass. Relatively high scanning speeds were used to produce a Doppler effect. In the one example given, the relative speed of the glass versus the detector was 650 centimeters per second.
U.S. Pat. No. 3,144,601 discloses a method for microwave detection of inhomogeneities in non-conducting materials such as glass sheets and plates. Detection was performed by simple measurement of the echoes of the reflected microwaves; by measuring losses in intensity following transmission through the object; or by mixing incident and reflected waves to create beats, particularly when the material being examined was traveling (i.e., detecting Doppler shifts in the frequency of the reflected microwaves).
U.S. Pat. No. 3,271,668 discloses the use of microwaves to measure the rate of progressive attrition from a surface of a body of a solid dielectric material; for example, measuring the burning profile in a solid rocket motor. Microwaves were transmitted through the fuel (or other material), the surface of which reflected some of the microwaves back to a detector. The relative phase of incident and reflected microwaves varied as the distance from the microwave transmitter to the surface of the burning fuel changed, allowing the distance to the surface of the fuel to be determined as a function of time.
U.S. Pat. No. 4,707,652 discloses a technique for detecting impurities in a bulk material by measuring changes in the scattering of microwave radiation incident on the bulk material.
U.S. Pat. No. 4,514,680 discloses a method for detecting knots in lumber, by transmitting microwaves through the lumber from two sources of the same intensity, but with a 180-degree phase shift. Transmitted microwaves are detected on the opposite side of the lumber. If the lumber is knot-free, there is a null in the microwave field at the detectors, but if a knot is present the phase and amplitude of microwave radiation at the detectors are altered.
U.S. Pat. No. 4,581,574 discloses a method for determining the average dielectric constant of a dielectric material having a conductive surface, by transmitting microwaves from two transducers into a sheet of the material, and making measurements of the energies of reflected microwaves. By measuring average dielectric constants along a plurality of paths in the plane of the sheet, locations of variations within the sheet may be identified.
U.S. Pat. No. 4,274,288 discloses an acoustic, interferometric method for measuring the depth of a surface flaw such as a crack.
U.S. Pat. No. 4,087,746 discloses a method for determining optical anisotropy in a dielectric material by measuring changes in the polarization of microwaves transmitted through the material.
U.S. Pat. No. 6,172,510 discloses the probing of targeted portions of a layered material by microwave radiation focused onto the targeted portion by adjustment of antenna position and orientation establishing a single oblique incidence path for reflection of antenna emitted probing radiation. Signal measurements of the radiation along the oblique incidence path are obtained to provide for evaluation and detection of defects in the targeted portion of the structure being probed.
A. Khanfar et al., “Microwave near-field nondestructive detection and characterization and disbands in concrete structures using fuzzy logic techniques,” Composite Structures Elsevier UK, vol. 62, pp. 335-339 (2003) discloses a near-field microwave nondestructive testing technique for disbond/crack detection and evaluation in a concrete structure. The frequency of operation and standoff distance could be optimized to achieve maximum sensitivity to the presence of a disband, which is viewed as an additional layer and which changes the properties of the effective reflection coefficient (phase and magnitude). The change depends on the thickness and location of the disbond. Multiple frequency measurements could be used to obtain disbond location and thickness information. A fuzzy logic model was described relating the phase of reflection coefficient, frequency of operation, and standoff distance to the disbond thickness and depth.
S. Ganchev et al., “Microwave detection of defects in glass reinforced polymer composites,” Proc. SPIE —International Society for Optical Engineering USA, vol. 2275, pp. 11-20 (1994) discloses the use of microwaves for defect and flaw detection in glass reinforced polymer composites. The standoff distance and the frequency were studied as means of increasing detection sensitivity.
A prior microwave method for the nondestructive testing of dielectric components employs virtual standing waves. See U.S. Pat. Nos. 6,359,446, 7,777,499, 6,653,847, and 8,035,400 These methods, while effective for detecting and characterizing thickness or density changes over a small range (plus or minus ¼ of the wavelength “λ” in the material being inspected), can give ambiguous results in some circumstances. Several different values for the thickness or density can correspond to a single value of the measured output. Despite the improvements represented by these earlier methods, the U.S. Pat. No. 8,035,400 patent frankly acknowledged: “There can be ambiguity in interpreting an interferometric signal, as points within the specimen that are spaced an integral number of half-wavelengths apart may not initially be distinguished from one another, due to the identical phase of the waves reflected from such points (where the wavelength in question is that within the material, which generally differ from the wavelength in air or vacuum, depending on the index of refraction).” One solution proposed was that “if a frequency is chosen to reduce the number of wavelengths needed to traverse the thickness of the specimen, one may enhance the sensitivity at a selected depth range with minimal ambiguity. In the special case where the specimen thickness is less than (preferably substantially less than) half the wavelength, then the imaging may be optimized for a single, very narrow band of the thickness within the specimen.” However, no solution was proposed for the more general problem of resolving these ambiguities when the thickness of the specimen can be several multiples of a wavelength. There is an unfilled need for improved testing methods that can resolve such ambiguities in measurements of bulk dielectric thickness, density, or features.
See also U.S. Pat. Nos. 5,539,322, 5,574,379, 5,216,372, 6,005,397, 3,025,463, 4,344,030, 4,754,214, 5,384,543, 7,190,177; Japanese patent abstract 61274209; and published international application WO9710514.